Normally 0.1 to 0.7 mass % of lead is added to zinc, anode active material for manganese dry batteries, purpose being to increase ductility for rolling and forming and corrosion prevention.
Lead deters cracks to run around the edges of a sheet in rolling ingot and on the wall of a can in forming the sheet.
It also acts to protect the can from electrolyte and corrosive impurities to corrode and to preserve a battery from deterioration due to self discharge.
So lead is a metal element effective both in process-ability and corrosion resistance as an additive for anode zinc. And because of its vital role, it has been thought almost impossible to exclude lead from anode zinc material for manganese dry batteries. However, lead is an environmental hazardous material, and supply of anode zinc material without additive of lead is seriously demanded and development is actively conducted thereof.
(Reference: JP6-196156A)
While development of anode active material without addition of lead has been conducted for some time, it is known that adding bismuth instead lead to pure zinc makes comparatively good corrosion resistance of the material and process-ability for rolling sheets and forming cans. (Reference: JP7-45272A)
The inventor conducted an experiment of the referenced technology by making an anode material and testing through the manufacturing process generally done. The process: making a bar of ingot by casting melted zinc alloy with bismuth into a mold and cooling down; rolling the ingot by a rolling mill to a zinc sheet in a given thickness; and punching the sheet with a press machine to a circular or hexagonal pellet to form a finished can. As a result; a can was made out anyhow; but progress of cooling was too slow and irregular or large rough crystals grew in the zinc sheet, which caused cracks around the rims of the zinc sheet; so material yield was quite poor and possible high rate of ‘No Good’ mixed in finished cans was feared for mass production.
The bismuth-added ingot could not be processed by a continuous casting and rolling system, which is the system for the lead-added ingot, because of breakage of molded sheets joining one another in band during extrusion and cracks at both sides before and after entering into the rolling mill. The range of processing parameters is narrower and optimization of manufacturing is extremely difficult for bismuth-added ingot, as compared for lead-added one. In fact, no example of successful manufacturing a zinc sheet exists from bismuth-added active anode material until today.
Conventional system detaching casting and rolling, where casting is followed by cooling in natural speed and then rolling proceeds, is impracticable either, because of low productivity for slow cooling, cracks, and defective finished cans; insurmountable disadvantage of productivity and cost.
As obvious from the described above, to realize manufacture and supply inexpensive and substantially lead-free manganese dioxide dry batteries, imperative is to solve the problems such as breakage, cracks, process parameters, and productivity involved in process by continuous casting and rolling system. That is one of today's most urgent and challenging task.